Abstract An analytical expression is presented for the susceptibility of liquid cast iron to solidify according to the Fe-C-X metastable system (also known as the chilling tendency of cast iron, CT). The analysis incorporates the nucleation and growth processes associated with the eutectic transformation. The CT is related to the physicochemical state of the liquid, the eutectic cells in the flake graphite, and the number of nodules in nodular cast iron. In particular, the CT can be related to the critical wall thickness, scr, or the chill width, Wcr, in wedge shaped castings. Finally, this work serves as a guide for understanding the effect of technical factors such as the melt chemistry, the spheroidizing and inoculation practice, and the holding time and temperature on the resultant CT and chill of the cast iron. Theoretical calculations of scr and Wcr compare well with experimental data for flake graphite and nodular cast iron.